This variable describes the SONET transmit clock source. Valid values are: loopTiming - transmit clock synthesized from the recovered receive clock. throughTiming - transmit clock is derived from the recovered receive clock of another SONET interface. localTiming - transmit clock synthesized from a local clock source or that an external clock is attached to the box containing the interface. external155MHz - transmit clock synthesized from an external 155.52 MHz source. The default value is 'loopTiming'. 'external155MHz' option is not valid for Sechtor 100.